As a conventional technology of this sort, there is one disclosed in JP-A-3-168339. In this conventional technology, a construction machine such as a hydraulic excavator is constructed with a traveling tilt-angle control device for controlling the tilt angle of a variable displacement travel motor, specifically a tilt-angle adjusting cylinder, a travel speed selector part, and a traveling, hydraulic pressure oil feed part. The travel speed selector part is comprised of a switch for selecting, as the revolution speed of the travel motor, either a first speed, in other words, a low speed or a second speed, in other words, a high speed. The traveling, hydraulic pressure oil feed part includes a switching solenoid valve, which is driven and controlled by a signal outputted responsive to a switching manipulation of the switch and can feed pressure oil to actuate the tilt-angle adjusting cylinder.
According to the conventional technology constructed as described above, while the switch is not in manipulated, the switching solenoid valve maintains closed a line, in other words, an oil passage that communicates a pilot pump and the tilt-angle adjusting cylinder with each other. Accordingly, pilot pressure oil from the pilot pump is not fed to the tilt-angle adjusting cylinder so that the tilt angle is maintained large and the revolution speed of the travel motor is maintained at the first speed, that is, at the low speed. When the switch is manipulated, the switching solenoid valve is actuated to open the oil passage. As a consequence, the pilot pressure oil from the pilot pump is fed to the tilt-angle adjusting cylinder, the tilt angle is decreased, and the revolution speed of the travel motor is changed to the second speed, that is, to the high speed.
As another conventional technology, there is one disclosed in JP-A-2006-292104. In this another conventional technology, a construction machine such as a hydraulic excavator is also constructed with a travel speed control unit, which is comprised of a traveling tilt-angle control device, a travel speed selector part, and a traveling, hydraulic pressure oil feed part. The traveling tilt-angle control device is comprised of a two-speed selector cylinder for controlling the tilt angle of a variable displacement travel motor. The travel speed selector part is comprised of a two-speed pedal for selecting either a first speed or a second speed as the revolution speed of the travel motor. The traveling, hydraulic pressure oil feed part includes a two-speed selector valve, which is driven and controlled by a signal outputted responsive to a manipulation of the two-speed pedal and can feed pressure oil to actuate the above-mentioned two-speed selector cylinder.
The latter conventional technology can change a preset relief pressure, and is provided with a relief pressure control part which can selectively maintain a maximum pressure of a delivery pressure of a main pump at a normal-time relief pressure or at a boost-time relief pressure which is a pressure higher than the normal-time relief pressure. This relief pressure control part is comprised of a main relief valve and a boost cylinder for actuating the main relief valve. Also provided is a boost-pressure switching selector part operably arranged to output a signal that can actuate the boost cylinder to change the preset relief pressure from the normal-time relief pressure to the boost-time relief pressure. This boost-pressure switching selector part is comprised of a detection circuit for detecting that the above-mentioned two-speed selector valve, which is included in the travel speed control unit, has been switched to a second-speed position. Further provided is a boosting, hydraulic pressure oil feed part including a boost valve that is driven and controlled by a signal outputted from the detection circuit and can feed pressure oil to actuate the above-mentioned boost cylinder. By the above-mentioned relief pressure control part, boost-pressure switching selector part and boosting, hydraulic pressure oil feed part, a boost control unit is constructed. It is configured such that, when the two-speed selector valve is switched to the second-speed position, the boost valve is actuated to change the preset relief pressure of the main relief valve from the normal-time relief pressure to the boost-time relief pressure, thereby making it possible to obtain a travel pressure higher than before.